1. Field of the Invention
The invention is related to the field of temperature sensors, and in particular, to a temperature sensor system that detects the temperature outside of the boundary layer of a mobile platform.
2. Statement of the Problem
Airplanes continuously sense the outside air temperature while in flight. When in flight, airplanes have a boundary layer that is formed by airflow around the airplane. The boundary layer typically extends about 3 inches above the skin of the airplane. The friction between the airflow and the airplane skin heats the air in the boundary layer, which is referred to as frictional heating. Thus, the air temperature within the boundary layer is artificially increased by the frictional heating.
To obtain an accurate outside air temperature that is unaffected by frictional heating, temperature sensors have been developed that attach to an airplane and extend outward beyond the boundary layer. Some of these sensors have redundant sensor components for reliability. Unfortunately, these temperature sensors do not have sufficient accuracy for scientific applications and optimal engine performance.
Because the temperature sensors extend away from the airplane through the boundary layer, the sensors introduce drag and increase fuel consumption. The extended temperature sensors reduce the stealth characteristics of the airplane. The extended temperature sensors also collect unwanted materials, such as ice and feathers, that cause sensor failure. Heaters are typically required for the extended sensors to prevent icing, but the heaters add cost, are subject to failure, and they can add errors to the temperature measurement.
An alternative temperature sensor includes a laser that directs a beam through the boundary layer. Energy from the beam is reflected from beyond the boundary layer and back to the temperature sensor. The temperature sensor processes the reflected energy to detect the temperature outside of the boundary layer. Although this alternative temperature sensor is more accurate than the above-described temperature sensors, the laser-based sensor does not provide accurate results in the presence of heavy fog, clouds, or precipitation that interfere with the laser beam and its reflection.